How to Measure Testes Size and Evaluate Scrotal Contents in the Stallion

1. Introduction 

Ultrasonographic (US) evaluation of the scrotal contents is used to evaluate the normalcy of the spermatic cord, testes, and scrotum. In addition, it is the most accurate and simple method for measuring testes size (i.e., volume), which is used to determine the efficiency of sperm production in the testes. Historically, stallion testes are measured using calipers. Calipers have the disadvantage of requiring the testis to be positioned so as to completely fill the scrotum. Often this requires manipulation of the testis by grabbing the scrotal neck to force the testis into a scrotal position. This type of manipulation can aggravate the stallion (i.e., kick). In contrast, the ultrasound can image and measure the testis in situ thereby minimizing excessive scrotal manipulation. Ultrasonographic testis measures are also more accurate since the parenchyma can be directly visualized and specific landmarks for measurement can be identified. Finally, US can be simpler and faster than the caliper method. 

Ultrasonographic evaluation of the scrotal contents in the stallion includes the spermatic cord (the spermatic artery, ductus deferens, and spermatic venous network (pampiniform plexus)), the epididymis (head, body, and tail), the testis, vaginal cavity, and tunics and scrotum (skin, dermis). The clinician should be able to identify position, location, and the normal echoic pattern of these structures. The examination of the scrotal contents, in addition to determining normalcy, should also include measurement of the length, width, and height of each testis. These measures are then used to calculate testis volume and determine the efficiency of sperm production.

Equipment

In general, linear array scanners with a 5.0-mHz probe can be used for evaluation of the scrotal contents and provide greater depth when measuring the length. Higher frequency probes provide greater detail but can be limited by the depth of penetration. Ideally, small sector scanner probes (Fig. 1) provide better access due to their smaller size and more specific identification of scrotal structures. The length measure of large testis may be greater than the limit of the US (e.g., 10–12 cm), and therefore that measure may have to be estimated. The T-type linear probe allows ease of handling and placement in the scrotal area.
 

Evaluation of the Testes and Scrotal Contents: Stallion Position and Location

1. Examine the stallion in a stall or quiet breeding shed area. Most stallions can be restrained in stocks if there are no other distractions to excite them. Administration of a light dose of detomidine hydrochloride^a (e.g., 0.007 mg/kgIV) may facilitate the evaluation.
2. An experienced stallion handler should be at the head of the stallion during the whole evaluation (Fig. 2).
3. Examine the stallion from the left flank area with the ultrasound positioned (Fig. 2) close to the evaluator but a safe distance from the stallion.
4. An initial cursory manual evaluation of the testes and scrotum should be performed to determine the degree of tractability and resistance that the stallion might exhibit (Fig. 3). The intent of this evaluation is to briefly evaluate size, position, and location of the testes and scrotal contents but not perform deep palpation since this may aggravate the stallion. Deep palpation should be performed as the final step in the evaluation.
5. Cover hands with disposable latex examination glove.
6. Apply a small amount (i.e., enough to allow probe-skin contact) of lubricant to the US probe surface. Gel can also be applied directly to the scrotal skin.
7. Once the clinician is comfortable with the stallion’s temperament towards the procedure, a position closer to the flank and scrotum can be taken.
8. Individual testis measures in centimeters (length, width, and height) can now be taken. Since the width and height are usually of similar size and easiest to measure, it is convenient to take these first. It is recommended to evaluate the near testis (left) first and take duplicate values for each measure.
   a. Height: The height is measured by placing the probe ventrally and directing the beam dorsally so that the central vein (Figs. 4 and 5) is approximately two-thirds of the distance from the surface and the spermatic artery can be visualized dorsally. There is no need to grasp or manipulate the testis for this measure. Note that the testis does not lie in a horizontal position but rather is tipped so that the cranial testis is higher than the caudal testis.
    

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